How To Measure For Moment Center

There are no bad race cars, just bad geometry. Any car can be made to work by making small changes to critical points in the suspension. That is why we present articles like this one--to show you how to improve the performance of your car by doing a little shop work and applying some cutting and welding skills to perfect your car’s design. It costs very little, but the rewards will be significant. The importance of the front moment center has not diminished even with newer setups that utilize bump devices and softer springs. The forces are still there that are influenced by the MC. We often write about the importance of the front MC location for stock cars. Now, we will again explain how to measure your car to find the exact MC location. Taking the measurements to determine the MC location is not that difficult. Racers in general will always do what is needed to perform, from making last minute engine changes to staying up all night to prepare for an important race. This process of measuring the front end takes from two to four hours, depending on how organized you are and how much help you have. This is considering that you will take your time and do it right by checking every measurement and verifying the data.

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The following are the steps and methods you can use to measure the eight pivot points in order to determine the locations of the front moment center. This represents a two dimensional view of the front geometry and is much quicker and easier than taking three dimensional measurements. If you desire to go the 3-D route, you will need to add measurements in a fore-aft direction, as well as the height and width, from a baseline that is perpendicular to the centerline. For most types of stock cars, the information we desire to know about the front moment center location can be found accurately enough by taking two dimensional measurements and using a two dimensional geometry software program. That is because the range of motion involved with negotiating turns on a short track add up to relatively small angular changes to the control arms.

Step 1
One of the most important steps is to locate a level portion of the floor on which to measure your car. The most important area that should be smooth and level is that area in between the front tires and extended fore and aft 10-plus inches. So, roughly a rectangular area that is about 20x70 inches and level will do. Pull a string over 1-inch blocks set at the outside and under where each tire will sit and across the area to be used. Measure along the string to see if the floor varies more than 1/8-inch in the exact spots you will be measuring to. If it does, move and find another area. Some teams will use a horizontal laser that can be purchased at any hardware store for this instead of a string. Get this part right and you will be able to trust your numbers.

Step 2
Position the car over the level spot with all of the weights in the car including the driver. Make sure to have the correct cambers set, the car at ride height, and the tires aired up to race pressures. If the engine has been removed, support the car under the framerails at normal ride height and remove the springs and shocks. Reduce the air pressures in the front tires until the LF lower ball joint flange drops 3/8-inch closer to the floor and the RF ball joint flange drops 1/2-inch closer to the floor. Dropping the spindles this way will closely simulate the amount that the tire squashes when all of the weight is on the tires with the engine, springs and shocks installed.

Step 3
Take a measurement at each front wheel so that we can later return the wheel assemblies to their same positions in relation to the chassis. We can measure at the outside of the tires from the bottom rim of the wheel to a mark on the fender or on the inside of the tire from the top of the ball joint stud to a point on the engine hoop bar. Some teams will measure the length of the shock. Whichever way, the idea is to have a way to accurately reposition the wheel assembly later on after we have raised the car and removed the springs and shocks.

Step 4
Jack the car up and support the chassis at the four corners on jackstands. Keep the stands away from the actual area where we will be taking the measurements in order to be able to access and measure to the pickup points under the front suspension without the stands being in the way. We want to raise the car the exact same amount at each corner. A good distance to raise the car is 10 inches, but any dimension will do. As we eventually measure the height of each chassis pivot point, the offset (amount we raised the car) will have to be subtracted from each vertical measurement and 10 is an easy number to subtract. Start by adjusting the lowest corner to the new offset distance, then move clockwise around the car to raise the other three corners to their ride heights plus the offset amount. Then make sure that the chassis is fully supported at all fours corners and does not rock and roll. You can shim the left rear corner last to stabilize the car and if the height is not perfect, we just leave that corner what it is. As long as the two front corners are set to ride height plus the constant offset and the chassis is tight on the jackstands, then you will be OK.

Step 5
Remove the front springs, shocks, and wheels and jack the LF and RF wheel assemblies up until they are positioned so that we have the same measurements as we took in step No. 3. Place a link in place of the shock at each side so that the wheel assembly is supported in the same position as if it were at normal ride height with all of the weight on the tires (step No. 3 measurements used). The link can be an old tie rod or tubular lower control arm with opposite threaded Heim joints of a usable length. This type of temporary link is easy to install and adjust for length. If those are not available, then a piece of 1x1/8-inch iron or aluminum metal strap can be used. Cut the piece in two so that they are long enough to overlap when installed in the shock mounts, drill a hole in one end of each piece and bolt the two to the shock mounts. With the wheel assembly positioned correctly, overlap the straps and lock them together with a pair of Vise-Grip pliers. Drill another hole through both pieces and bolt the two together. Label each set of straps, i.e. LF, RF so that you can reuse them if you need to re-measure later on.

Step 6
Establish and mark the center of rotation of each ball joint. It is important to know the exact location of the pivot point of the ball joint so that we can mark that point on the ball joint support band on the control arms. To find the center of each different type of ball joint, we can quickly locate it using a simple method of placing the ball joint in a vise with the stud pointed up and then lining up the shaft as it moves from side to side. Now that we know where the center of rotation is located in relation to the band, we can mark a point on each ball joint on the car. Remember to allow for antidive angle in the control arms as well as the control arm angle. These angles will affect where you place the point to measure to. Use 3/4- or 1-inch wide masking tape and place a piece over the ball joint band. Clean the surface first to remove all grease and dirt. Use a fine tip black marker and a small straight-edge to make a cross on the tape to represent the center of the ball joint for height and width measurements. Do this for the upper and lower ball joints.

Step 7
Take all of the height measurements first. If a direct vertical measurement cannot be taken, then use a level to project the height out from each point or construct a fixture to use to represent the distance so that a measurement can be taken away from the car. When measuring the lower chassis points, use the pivot that is closest to a line lying at right angles from the ball joint to the centerline of the car. For the stock GM lower control arms, use the center of the front bushing. For a strut type of lower control arm, use the center of the bushing or center of the Heim joint that is on a line lying 90 degrees off the centerline of the car projected through the center of the ball joint. The upper chassis mounts often have antidive. To measure the chassis pivot point, average the heights of the centers of the two mounting bolts and measure the width to the center of the control arm shaft. In the case of an upper control arm that uses Heim joints and links, measure to the pivot that is closest to a line that would be lying perpendicular to the centerline of the car from the center of the ball joint.

Step 8
Establish a centerline for the purpose of moment center location. This will not be the true centerline of the chassis, but rather a point half way between the front tire contact patches. Note that the car “feels” the MC in relation to the tire contact patch points and therefore we need to know the relationship of its location to those tire patches. After placing the wheels and tires back on the car, mark a point on the floor at each outside edge of the front tires. I often hang a plumb bob over the bulge in the tire to the floor. Measure between these points and divide that measurement by two. Place a mark on the floor between the front tires that represents half the distance between the tires. Using that same half-distance measurement, measure from the outside of the RR tire to mark a rear centerline point. The right side tires are supposed to be inline or very close to it, and so our points will be parallel to the right side tire patches and centered between the front contact patches. If you know your RR tire offset, you can add or subtract from the front half distance for the rear measurement. Stretch a string over these two centerline points, pull tight and hold each end with blocks of lead or a concrete block if no lead is available. We will be measuring from this line to each of our eight pivot points.

Step 9
Now it’s time to measure the widths. Drop a plumb line down from each center of rotation for the four ball joints and the four chassis pickup points and place a mark on the floor. The framerail and lower control arms may prevent you from dropping straight down from some of these points. In that case, measure out beyond the spindle at a right angle to the centerline and plumb down and mark an offset point on the floor. Write the offset amount (use 20, 30, and so on inches) on the masking tape beside the point. The point we will measure from at each ball joint must be either at the front or rear of the BJ so the measurement will be an accurate width dimension. When marking these points on the side of the ball joint, make sure you are looking either directly to the front or rear of the car.

Measure from each point to the centerline and don’t forget to subtract the offsets.

Step 10
Once all measurements have been taken and recorded, enter all of the height and width measurements into a racing geometry software program. Check to make sure that each measurement is entered correctly. Try to think out the difference in the numbers so that they make sense. If the inner points of the upper control arms are higher than the ball joint measurements, and you know the physical angle shows the ball joint to be higher, re-check your measurements. We are interested in finding the location of the MC in both static and dynamic positions. Static represents where the MC is located when the car is at static ride height and the dynamic position is where the MC migrates to as the car dives and rolls in the turns. The dive and roll numbers you will enter into the program are in direct relation to the type of car, track banking angle, and setup stiffness. Try to use dive numbers that represent what the center of the car is doing at mid-turn and the roll angle is how many degrees the chassis is rolling relative to the track surface.

Low banked tracks will produce more roll angle and less dive than a high banked track. If you’re running on bump devices or into coil bind, then simulate where the chassis would be relative to that attitude and put those numbers in dive and roll in the program. These numbers must make sense and simulate the attitude of the car at mid-turn. Shock travels may include additional travel distance as a result of hard braking on entry into the turns and may not accurately represent the mid-turn attitude. Review the past Circle Track articles on moment center design to determine if your moment center location is correct for your application. The design of the location of the moment center is critical to how your front end will work and is an important ingredient in determining the overall balance of the setup in your car.